This invention relates to a peak-referenced-threshold detecting type proximity detector, and especially to a ferrous-gear-tooth transducer including a Hall element and a Hall voltage amplifier, and more particularly relates to such a transducer in which the transducer output voltage signal peaks are automatically centered within the dynamic range of the detector.
The term "magnetic article" as used herein applies to magnetized bodies, ferrous bodies and other bodies having a low magnetic reluctance that tend to alter the ambient magnetic field.
In the patent U.S. Pat. No. 5,442,283, issued Aug. 15, 1995 there is described an integrated circuit Hall-voltage proximity-detector, which employs a Hall-signal peak-referenced-threshold detector. The integrated circuit chip is mounted to a pole of a magnet. A peak-referenced-threshold type of signal detector tracks a slope of a Hall voltage (e.g. corresponding to the approach of a passing gear tooth) and briefly holds the ensuing peak voltage before producing an output signal indicating the onset of the following Hall-voltage slope of opposite direction (e.g. corresponding to the approach of a valley between two gear teeth).
In this patent, the Hall voltage holding circuit includes a capacitor and a circuit means for controllably leaking charge out of or into the capacitor for preventing false tripping of a comparator that provides the transition in the binary output signal. The holding voltage of the capacitor thus has a droop which leads to increasing loss of holding accuracy as the speed of gear tooth passage becomes slower, and therefore the detector has a minimum gear teeth speed at which accurate detection is possible. A capacitor used in the engine compartment of an automobile is typically required to operate over the temperature range of -40.degree. C. to 170.degree. C.
When a peak-referenced-threshold detector of the kind described in the patent U.S. Pat. No. 5,442,283, is so used, the associated peak holding capacitor must have a very high leakage resistance to prevent missing the correct cylinder-firing times at start-up (crank start). Even use of such an expensive capacitor would cause the missing of the first one or two firing times.
Most proximity detectors of the prior art produce a high binary output voltage indicating approach and proximity of a passing article, and produce a low binary voltage when the article recedes from the detector. The signal detector is conventionally of a type that may be called the mid-signal referenced threshold detector, or just the mid-signal detector type, wherein the transition in detector output voltage from low to high typically is triggered by a comparator that determines when the transducer voltage rises to (or falls from) a fixed internal threshold voltage referenced to a voltage level corresponding to the median or average value of the Hall signal.
Alternatively, in the case of the peak-referenced-threshold detector in the above-described patent, the detector output transition occurs when a transducer voltage peak has just occurred and the transducer signal voltage has dropped by an amount equal to a predetermined threshold voltage from the peak value.
Proximity detectors having fixed threshold voltages, produce low to high (or high to low) binary transitions in the output signal indicating approach of a magnetic article. In practice, the closest passing distance (sometimes referred to as the air gap) does not remain constant.
Variations of the air gap dimension causes shifts in the actual distances of article approach and receding at which the transducer voltages exceeds or falls below the fixed thresholds. This results in a lack of accuracy of passing detection that may rule out their use as position detectors of passing articles such as cams and gear teeth.
Changes in the air gap, between passing articles to be detected and the transducer, may be attributable to mechanical and electrical properties of the detector as well as in the properties of the passing articles, especially as a function of temperature.
Other causes of inaccuracy stem from the fact that the amplitude of the Hall voltage changes when gear teeth (magnetic articles) have different ferro-magnetic properties from tooth to tooth, and/or when A undulating changes in the spacings (air gap) of gear teeth to detector are caused by eccentricity of the gear. Also, changes in temperature cause changes in air gap dimensions and in the sensitivity of the transducer and transducer-voltage amplifier.
Whether detection is accomplished by sensing the Hall voltage peaks or using a voltage threshold criteria for indicating approach of a passing article, changes in the median amplitude of the transducer voltage degrade the accuracy of position detection because the Hall signal is not centered within the dynamic range of the Hall signal detector.
It is an object of this invention to provide a proximity detector having a magnetic-field-to-voltage transducer and providing automatic centering of the transducer output voltage within the dynamic range of the transducer-signal detector.
It is a further object of this invention to combine automatic gain control with the automatic centering of transducer output voltage within the dynamic range of a capacitor-less transducer-signal detector of the peak-referenced-threshold type, to provide great accuracy of detection down to zero speed.